1. Field of the Invention
This invention relates generally to the field of gas turbine engines for the generation of electricity and, more particularly, to a method for ignition and start up of a turbo generator.
2. Description of the Prior Art
The starting of a gas turbine engine is a complex operation. Typically, before the gas turbine engine can run on its own power, the engine must be accelerated by an external source, such as a battery, to provide sufficient airflow to the combustor for ignition, typically referred to in the industry as light-off. In a turbogenerator having a permanent magnet motor/generator coupled to a gas turbine engine, supplying electrical power to the permanent magnet motor/generator will function as a motor to drive the gas turbine engine. Typically, engine speed varies as a function of the torque versus speed characteristics of the starter motor.
The combustion air to the combustor increases generally with gas turbine engine speed. Ignition occurs when the speed of the engine produces enough combustion air to produce the correct ratio of air with the fuel supplied. In order to obtain this correct fuel-to-air ratio, the amount of fuel flow to the gas turbine engine is actively controlled as a function of the speed of the gas turbine. However, fuel flow is highly dependent upon ambient conditions, such as atmospheric pressure and temperature.
Typically, in order for the correct fuel-to-air ratio to be achieved for light-off, atmospheric pressure and temperature must be accurately known or, otherwise, the representation of fuel flow will not be accurate and throw off the fuel-to-air ratio. In addition, any deviation in the measurement of gas turbine engine speed, or in the correlation of combustion air with gas turbine engine speed, can easily throw off achieving the correct fuel-to-air ratio for ignition. Therefore, in a prior art starting procedure where speed (combustion air) and fuel flow are variable, ignition is not attempted until the correct fuel-to-air ratio is thought to have been achieved, generally through experience.
In another prior art starting procedure for gas turbine engine ignition, the gas turbine engine operates at a fixed speed to provide a substantially constant supply of combustion air for light-off, while the fuel flow is variable. The fuel flow is then ramped up to achieve the correct fuel-to-air ratio, at which point ignition occurs. This fixed speed method is insensitive to fuel control variations, gas turbine engine variations, and ambient conditions.
In any starting method for a gas turbine engine, after ignition occurs, the accelerator rate of the turbine engine increases rapidly thereby increasing the air flow. The exhaust gas temperature also increases rapidly indicating that light-off has occurred. A controlled accelerator rate of the gas turbine engine after ignition provides cooler exhaust gas temperatures resulting in consistent and smoother start up. A smoother start up helps prevent over-temperature and saves energy.
It is, therefore, an object of the present invention to provide a consistent and controllable method for ignition and start up of a gas turbine engine.
The present invention is a method for ignition and start up of a gas turbine engine that includes the following steps. First, ambient conditions and fuel heating value is determined. The ambient conditions can include ambient temperature and atmospheric pressure. Second, the gas turbine engine is accelerated at a preset acceleration rate to a speed to provide a supply of combustion air to the combustor for ignition of the gas turbine engine. The preset acceleration rate of the gas turbine engine is determined based upon the ambient conditions and the fuel heating value. Acceleration of the gas turbine engine is achieved by applying electrical assistance, such as battery power, whereby a turbine drive shaft rotates causing compressor blades and turbine blades to rotate. Third, an ignition source, such as an igniter, is activated in the combustor of the gas turbine engine based upon the ambient conditions and the speed-of-rotation of the compressor blades and the turbine blades of the gas turbine engine. Fourth, a flow of fuel is supplied to the combustor at a substantially constant rate to provide an optimum supply of fuel to the combustor. The supply of fuel at this constant rate is maintained until the correct fuel-to-air ratio is achieved and ignition of the gas turbine engine occurs. After initiating the flow of fuel at a substantially constant rate, the fuel flow is increased non-decreasingly until the engine drives a load. Fifth, ignition of the gas turbine engine is sensed by using a heat sensor located adjacent the turbine exhaust gases to measure exhaust gas temperature of the gas turbine engine. Sixth, the acceleration rate of the gas turbine engine after ignition is controlled. Seventh, electrical assistance to the gas turbine engine is eliminated based upon a fixed value of the rotational speed of the turbine blades of the gas turbine engine. This fixed value is determined by the torque versus speed characteristics of the starter motor. Eighth, the flow of fuel to the combustor is ramped up to increase the speed of the gas turbine engine to a final speed to which a load can be applied. Finally, the acceleration rate of the gas turbine engine after the flow of fuel is ramped up to the combustor is controlled.
The combustor can also have a multiple number of fuel orifices. The fuel orifices can be in fluid communication with either a single fuel source or multiple fuel sources. However, in a situation where the combustor is idled at an idle speed, fuel can be supplied to the combustor at a constant flow rate, so that the combustor is operating at a slower idle speed without any electrical assistance to the gas turbine engine.
In a situation where ignition does not occur after a fixed period of time at the constant flow rate of fuel, the fuel flow rate is increased until ignition does occur. If ignition is still not achieved after a specified period of time, the system is stopped and purged.